The electromagnetically induction-heating type fixation device is, for example, configured to generate a magnetic flux by applying a high-frequency pulse to an exciting coil so as to induce a material to generate heat. Unlike a heat roller type fixation device, since the electromagnetically induction-heating type fixation device having such a configuration is capable of inducing the material to generate heat by itself, the fixation device does not require preheating. That is, the fixation device of this type can instantly increase the temperature of the material to a predetermined level. Accordingly, the fixation device of this type can reduce warm-up time to allow for energy conservation. The fixation device of this type often uses the maximum power thereof in a power control mode at the start-up to save warm-up time.
However, the fixation device of this type is designed to have small heat capacity, and can therefore raise its temperature to a target temperature in several seconds if the fixation device starts up with a certain preheated temperature in a preheating mode. However, this electromagnetically induction-heating type fixation device may cause, after having reached the target temperature, temperature overshoot that exceeds the target temperature of the fixation device. The temperature of the fixation device after temperature overshoot may not be stabilized until several seconds have elapsed. Accordingly, inconsistent gloss or hot offset may be observed in images of several sheets (pages) from the first sheets starting to pass through the fixation device, due to fixation of toner on the printing sheets on the fixation belt having the unstable temperature.
To overcome such a drawback, a technology is proposed to control the amount of current applied to the exciting coil based on a preheated condition of the fixation device before the application of current to the exciting coil. For example, Japanese Laid-Open Patent Application No. 2005-257945 discloses a technology in which a fixation device is initially activated in a power control mode to apply the current to an exciting coil, and when the temperature of an image heating member (i.e., fixation belt) reaches a certain temperature (i.e., difference between fixation belt temperature and image fixation setting temperature), then the power control mode of the fixation device is switched to a temperature control mode. More specifically, in the disclosed fixation device, a control value (proportionality coefficient Kp) of PID control in the temperature control mode is changed based on a previously accumulated heat of the fixation belt before the application of current to the exciting coil. That is, if the temperature of the image heating member (fixation belt) is equal to or more than the default value before the application of current to the exciting coil, the control value of the PID is reduced. In this manner, the temperature of the fixation belt, after having reached the target temperature thereof, can be prevented from overshooting. In contrast, if the temperature of the fixation belt is lower than the default value before the application of current, the control value of the PID is increased. In this manner, the ascending speed of the temperature of the fixation belt can largely be increased.
In addition, Japanese Laid-Open Patent Application No. 2006-058732 discloses a technology in which whether to apply the power from an auxiliary power unit (capacitor) is determined based on the temperature of the fixation device before the application of current to an exciting coil. If the temperature is equal to or higher than a threshold temperature, the current is not applied to the exciting coil. In this manner, the temperature of the fixation device, after having reached the target temperature thereof, can be suppressed from overshooting. In contrast, if the temperature of the fixation device is lower than a threshold temperature, the current is applied to the exciting coil. In this manner, the ascending speed of the temperature of the fixation device can largely be increased.
However, in the technology disclosed in Japanese Laid-Open Patent Application No. 2005-257945, even if the power control mode of the fixation device is switched to the temperature control mode when the temperature of the fixation belt reaches the temperature obtained by the difference between fixation belt temperature and image fixation setting temperature, the overshooting of the target temperature of the fixation belt (image heating member) may not be sufficiently suppressed to prevent image degradation. Further, in the technology disclosed in Japanese Laid-Open Patent Application No. 2006-058732, even if the power from an auxiliary power unit is not supplied to the fixation device, the fixation device at start-up is consistently operated in the power control mode. Thus, the overshooting of the target temperature of the fixation belt at the start-up of the fixation device may not be suppressed.
In the technology in Japanese Laid-Open Patent Application No. 2005-257945, the fixation device is initially activated in the power control mode and then the power control mode is switched into the temperature control mode. However, time required for switching the power control mode to the temperature control mode while the fixation device is in the power control mode requires a full control period. Moreover, the power of the fixation device may not instantly drop. Note that the speed of increasing temperature in the induction-heating type fixation device is extremely fast. Accordingly, if a pressure-application member is configured to store heat, the temperature rise speed of the fixation belt is too fast to suppress the overshoot temperature by adjusting the control value of the PID control in the temperature control mode. Therefore, the overshoot temperature of the fixation belt may not be prevented. In other technologies, the start-up speed of the fixation device is more focused than the overshoot temperature of the fixation member, so that the fixation device starts up in the power control mode. However, as the melting point of toner is increasingly made low, wax in toner vaporizes (ejects) and the vaporized wax is attached to members in the vicinity of the nip portion. Since accumulation of such wax attachments degrades image formation, the overshoot temperature of the fixation member needs to be suppressed.